R is the need-to-know stat software
This Forbes post on the greatness that is R is being passed around by every statistician and his mother today.
It's not that this type of analysis wasn't possible before — statisticians have existed, and commercial software has been available to support them, for decades. The fact that R is free to use, free to modify, and its source is open to view, extend and improve means students, stock traders-in-training and fantasy football junkies can familiarize themselves with the software. They can write programs against it. They're likely to continue that usage into their professional lives. When they share their work, the community, down the line, benefits. And the virtuous cycle strengthens.
What's your favorite (graphical) use of R?